Cape Town looks towards desalination

The City of Cape Town plans to call for tenders for a feasibility study on a large-scale seawater desalination plant, as the region runs increasingly close to using all of its available fresh water supply.

Water scarcity may require winter rainfall pumped from the Berg into Voelvlei Dam

The tender call, expected within a month, will be for a study on where such a desalination plant could be built and what capacity it should have.

The call coincides with a major effort to plug water leaks and theft that, in February last year, accounted for one quarter of all treated water in the city, and with a warning that few options remain for tapping existing surface water sources.

The city will also be looking at the large-scale re-use of water. This is the only potential major new water source at a cost lower than seawater desalination, which is very expensive because of the large amount of electricity required. This study is expected to kick off “within the next few months”.

These initiatives are among the water conservation and water demand management measures that form a major part of the strategy for providing water in the Western Cape region that is already using about 92 percent of all “safely” available water. “Safely” means with a high degree of certainty of availability, without water restrictions.

Depending on how successful these measures are and on how much the city grows, the remaining 8 percent of available water will be fully utilised anywhere between 2017 and 2019, according to projections by the Department of Water Affairs.

In a statement yesterday based on the latest newsletter of the Western Cape Water Resource Strategy’s steering committee, the department said it was therefore supporting the city’s efforts by calling on all residents to use water sparingly.

It pointed out that the Western Cape Water Supply System could safely provide 556 million cubic metres a year. Users of this system include the municipalities of Stellenbosch, Drakenstein, Swartland and Saldanha, as well as farms.

Last year, 511 million cubic metres were used, with 32 percent going to irrigation farmers and 68 percent to urban residents.

The department pointed out that the city had invested some R60m in the current financial year to upgrade and replace water infrastructure, metering, water pressure management and other measures to reduce water loss and the amount of “non-revenue” water.

“Non-revenue” water is water that goes missing in the distribution system between the collection point and the metered outlets, as a result of pipe bursts or leaks, reservoir overflows, metering inaccuracies and theft.

“(This investment) has had the effect of reducing the average ‘lost’ water, in relation to the water treated and supplied, from 24.4 percent in February last year to 19.9 percent in February this year.”

The department said it was looking at the feasibility of two surface water options for increasing supply: pumping winter rainfall run-off water from the Berg River into Voëlvlei Dam and diverting winter rainfall above an agreed threshold from Mitchell’s Pass (Ceres) in the Dwars River-Breede catchment, to the Klein Berg River and from there through existing diversion works also into Voëlvlei.

Both options would make additional water available in the Western Cape Water Supply System and for the West Coast District Municipality.

The downstream ecological water requirements of both schemes were being determined. In terms of the National Water Act, rivers must retain sufficient water – the “ecological reserve” – to allow proper ecosystem functioning.

The department said it was also involved in a study to see whether the region’s water supplies could be augmented by the “further artificial recharge” of the Langebaan aquifer, as the city council does successfully with the Atlantis aquifer.

“A recent study by the West Coast District Municipality to augment the Langebaan Road aquifer by means of injecting surplus winter water from the Berg River into the aquifer for use during the summer months has not been as successful.

“What was thought to have been a confined aquifer unfortunately seems to be ‘punctured’ by over-abstraction and boreholes… into the lower aquifer system.

“Further studies will be done, which will include the possibility of closing off all boreholes penetrating the Lower Langebaan Road Aquifer System.”

It also stressed that the removal of invasive alien vegetation was becoming “increasingly urgent”, as water used by these plants could be much better used.